Silicon carbide (SiC) has superior semiconductor characteristics and, compared with the conventional materials of silicon (Si), gallium arsenic (GaAs), etc., is particularly remarkably superior in heat resistance, insulation breakdown field, etc., so is being noted as a wafer material for power devices, high frequency devices, and various other types of semiconductor devices. As devices using SiC single crystal wafers, GaN-based blue light emitting diodes, Schottkey barrier diodes, etc. have already been commercialized. Further, in addition, this is being used for production of prototypes as a substrate material for GaN-based high frequency devices and low loss power devices such as MOSFETs.
At the present time, SiC single crystal ingots having a large size suitable for production of devices are generally being produced by sublimation recrystallization based on the improved Rayleigh method (Yu. M. Tairov and V. F. Tsvetkov, Journal of Crystal Growth, vol. 52 (1981) pp. 146). This sublimation recrystallization is based on 1) using an SiC single crystal wafer as a seed crystal and charging a graphite crucible with a material comprised of SiC crystal powder, 2) heating this in an argon or other inert gas atmosphere (13.3 Pa to 13.3 kPa) to a high temperature of about 2000 to 2400° C. or more, and 3) arranging the seed crystal and material powder so as to form a temperature gradient where the seed crystal becomes the lower temperature side compared with the material powder. Due to this, the sublimating gas produced from the material is diffused and transported in the seed crystal direction. Single crystal growth is realized by recrystallization on the seed crystal.
For device applications, control of the electrical conductivity characteristic of the wafer material in accordance with the application is necessary. For this purpose, it is necessary to establish technology keeping to a minimum the impurity concentration in the crystal so as to produce high purity single crystal as is the practice with production of Si, GaAs, and other conventional semiconductors. This is becoming important basic technology for enabling precision control of the electrical conductivity characteristic of single crystal wafers. In particular, the impurity element able to form a donor or acceptor has a great effect on the electrical conductivity characteristic of the single crystal. The dosage or mixed amount of the impurity has to be strictly managed.
In the case of a SiC single crystal, nitrogen and boron can be mentioned as typical elements corresponding to the above impurities. While depending on the polytype of the SiC single crystal, in the large forbidden band of SiC crystal extending up to 2.5 to 3.0 electron-volts (eV), nitrogen forms a donor level at the shallow position, while boron forms an acceptor level at the shallow position, so ionization is easy even at room temperature. The electrons or vacancies emitted from the nitrogen or boron atoms into the crystal become carriers and act to push up the electrical conductivity. Therefore, when, like in high frequency device applications, an extremely low electrical conductivity is required from the SiC single crystal wafer, the concentration of boron or nitrogen in the SiC crystal has to be reduced to at least 1×1017 cm−3 or less.
However, even if reducing the impurity concentration, carriers remain corresponding to the difference in the concentration of the impurity forming the acceptor level and the concentration of the impurity forming the donor level. Sufficiently low electrical conductivity cannot be reached by just these mechanisms. Therefore, to obtain a sufficiently low electrical conductivity, the introduction of impurities forming deep levels and atom vacancies etc. is being studied in semiconductor crystals. Various studies have been made on SiC crystals as well.
In particular, atom vacancies can form deep levels without relying on the addition of impurities, so this is considered a preferable method in terms of securing crystallinity and low electrical conductivity. However, the specific types of atom vacancies contributing to electrical conductivity had not been identified. Therefore, it had been necessary to identify and introduce the type of vacancies involved in electrical conductivity to secure a low electrical conductivity without relying on the addition of impurities.
To realize a sufficiently low electrical conductivity, that is, semi-insulation, it is considered necessary to satisfy |nD−nA|<nV from the relationship between the vacancy and carrier concentrations. Here, nD, nA, and nV are the donor concentration, acceptor concentration, and vacancy concentration involved with the electrical conductivity. To satisfy this equation, both the donor concentration and acceptor concentration have to be reduced. Reduction of the nitrogen impurity concentration contributing to the donor concentration and the boron and aluminum impurity concentrations contributing to the acceptor concentration is being sought. However, in practice, the nitrogen impurity and boron impurity easily enter from the SiC material, graphite crucible, etc., so reduction of the impurity concentration is difficult. In particular, nitrogen is also included in the atmosphere, so more easily enters. In the impurity concentration in SiC single crystal, the nitrogen impurity concentration is often higher than the boron impurity concentration and as a result nD−nA<nV must be satisfied. This situation means that the type of the vacancies also has an effect.
That is, since the donor concentration is high, among the various types of vacancies, the condition is added that the charged state of the vacancies be negative, that is, that electrons can be accepted. In addition, as a condition sought for the type of the vacancies, in particular thermal stability is important. The reason is in part that depending on the type, the vacancies are greatly decreased by heat treatment. Heat treatment is necessary in the process of forming the various types of films required for formation of the devices on an SiC single crystal wafer. For this reason, in preventing change of the electrical conductivity of the wafer due to heat treatment, it has also been strongly desired that the vacancies be of a type difficult to decrease due to heat treatment. In addition, since it is known that depending on the type, vacancies are greatly decreased by heat treatment, thermal stability has been important as a condition required in the type of vacancies.